1. Technical Field
The present invention relates to a lens device which is suitable for broadcast or movie and a position detection method of a movable optical element mounted in the lens device.
2. Related Art
Recently, due to the advancement of screen enlargement and definition enhancement in televisions or monitors, a demand on a higher quality of an image to be displayed is increased. In order to satisfy the demand on the higher quality image, a position detector (an encoder) capable of detecting a position with high precision is mounted in a zoom lens for broadcast or movie so as to achieve the high performance of a lens control.
Hitherto, encoders disclosed in Patent Literature 1 (JP-A-4-307327) and Patent Literature 2 (JP-A-5-099688) have been suggested as a compact and low-cost encoder.
In the encoder disclosed in Patent Literature 1, a single track provided on a rotary drum is divided into 2n regions, and in each region, an absolute position code which indicates an absolute position and identification codes provided at both ends of the region so as to identify the region are magnetized. Further, when the identification codes are detected by an MR sensor, the identification codes and the absolute position code detected between the identification codes are adapted to be output.
The encoder disclosed in Patent Literature 2 has a single track provided on a rotary drum and divided into a plurality of regions, and in each region, an absolute position code which indicates an absolute position is magnetized. The absolute position code is subjected to frequency modulation and magnetized such that if the absolute position code is a decimal digit “1,” the magnetization frequency is 100 Hz; if the absolute position code is a decimal digit “0,” the magnetization frequency is 50 Hz; and if the absolute position code is a decimal digit “2,” the magnetization frequency is 150 Hz.
In the encoder disclosed in Patent Literature 2, no consideration is given as to how to detect a start position of the absolute position code.
Meanwhile, according to the encoder disclosed in Patent Literature 1, since the identification codes exist, the start position of the absolute position code may be readily detected.
However, in the encoder disclosed in Patent Literature 1, it is necessary to provide a non-magnetized region at least by the absolute position code between identification codes of different regions. For this reason, after the position of a certain region is detected, it is necessary to rotate the rotary drum at least by one region until a region adjacent to the certain region is detected.
For this reason, when the encoder disclosed in Patent Literature 1 is applied to a lens device, since it is necessary for a user to rotate a lens barrel greatly, for example, until a lens position is detected after power is turned on. Thus, use convenience is deteriorated.
Further, since a single absolute position is detected from a region corresponding to two absolute position codes, it is difficult to increase the number of divided regions in a single track. In a lens device, in particular, in a business purpose lens device or the like, the rotational position of the lens should be finely detected. Thus, an application to the lens device is difficult in the technique of Patent Literature 1.